1. Field of the Invention
The invention relates to calibration of a fuel control valve in an injector for an engine fuel injection system.
2. Background Art
Control valve assemblies for fuel injector pumps are designed typically to have a fuel delivery rate and engine crank angle relationship that will achieve an optimum level of engine exhaust gas emissions. Engine emission standards require control of the fuel quantity and timing of the fuel injection at the combustion chamber to match the engine cycle. Effective fuel injection rate shaping will result in a reduced level of oxides of nitrogen and a reduced level of particulates in the engine exhaust gases. Effective rate shaping also affects engine operating efficiency and engine noise.
U.S. Pat. No. 6,158,419 discloses an example of a control valve for an engine fuel injector wherein the actuator for the control valve is capable of shaping the injection rate. This patent is assigned to the assignee of the present invention.
The injector pump of the ""419 patent comprises a fuel pumping chamber located in a pump body, and a valve chamber between the pumping chamber and the fuel delivery nozzle. The nozzle delivers fuel under pressure to the combustion chamber of the engine. A valve seat is formed in the valve chamber. A valve in the valve chamber has an axially extending guide portion, which controls fuel delivery past the valve seat and into the injector nozzle portion of the system. The valve also has a sealing surface that is movable in the valve chamber between a valve closed position and a valve open position. When the valve is in the closed position, the valve sealing surface engages the valve seat. In the open position, the valve sealing surface is spaced from the valve seat. The valve has a stepped portion that extends a limited distance from the sealing surface, which provides a limited pressure relief as an injector pumping piston is stroked.
A valve spring urges the valve toward its open position. An electromagnetic actuator urges the valve toward its closed position against the bias of the valve spring.
An injector that would be calibrated in accordance with the invention would include a valve that has a fuel injection rate shaping feature. By varying the amperage for the valve actuator, rate shaping can be achieved without the necessity for modifying the injector assembly, or modifying the output pressure before the pressurized fuel reaches the injector nozzle, or modifying the nozzle itself to control the nozzle spray pattern. Injection pressure control is used instead of throttling the fuel flow at the nozzle to achieve effective rate shaping.
Controlled pressure relief by the valve accommodates a small amount of dimensional tolerance for obtaining an intermediate position of the spool valve so that the control valve may achieve, within a calibrated range of positions, an optimum rate shaping characteristic. This rate shaping is used near the beginning of the injection event before the top-dead-center position of the engine piston.
The disclosure of the ""419 patent is incorporated herein by reference.
The invention makes it possible to calibrate a fuel injector by establishing a so-called boot current level for the control valve actuator. Dimensional tolerances and other variables in the design and construction of fuel injectors for internal combustion engines make it necessary to individually calibrate each fuel injector for each cylinder of a particular engine with which the injectors are used. The calibration process includes a series of steps that comprises the present invention.
In practicing the method of the present invention, a boot current level is initially established based on prior experience. The injector is then tested with that boot current level, and the stability of the boot phase of the injection event is evaluated. If stability is confirmed, then a search algorithm is started to find the limits of the boot current level.
This test is done typically at two engine speeds, such as 650 rpm and 900 rpm. The method steps of the present invention make it possible to establish the upper limit and the lower limit for the boot current at each engine speed. The tests further will determine where within the calibrated upper and lower limits the boot current level of a particular injector will fall. A boot current in excess of the upper limit may result in injector instability. Similarly, a boot current that is lower than the lower limit will result in an unstable injector. Injector boot instability will result in poor engine performance, power and emissions.
The boot current level is incremented up or down at each step in the calibration method. The increment becomes smaller until a reliable limit is found. Any boot current level that will develop an unstable boot pressure (i.e., one falling outside the limits placed on the calibrator) will result in poor engine performance and emissions.
The determination of the low limit and the high limit for the boot current makes it possible to calculate a set point value. That set point value is corrected using an empirical correction based on observed differences between behavior of the injector on the injector calibration stand and the behavior of the same injector when it is mounted on a given engine. Thus, the calibrated boot current that is determined using the present method is not necessarily the algebraic average of the high value and the low value. The calibrated boot current established using the present calibration method will fall, however, within the upper and lower limits.
In practicing the method of the invention, the injector is calibrated by choosing an initial boot current level, as previously mentioned, and then incrementing the initial boot current several times. The increment is progressively decreased in successive steps, each step being followed by a determination of whether the corresponding boot current is too low or too high to maintain injector boot pressure stability. The final boot current determined in the final step is used to calibrate the boot current set point which is delivered to the engine controller as a coded value during the engine assembly process. The information may be transferred to the engine ECU in many ways including bar coding, human read and manually entered, by association to a database, etc.